In the field of data transmission in which digital word sequence data containing information data representing signal information such as video signal information or the like are transmitted, an electric transmission system and an optical transmission system have been proposed to be put into practice. In the case of the electric transmission system, the digital word sequence data are converted to serial data and one or more electric signals are produced based on the serial data to be transmitted through one or more transmission lines each made of a coaxial cable or a pair of twisted lines. In the case of the optical transmission system, the digital word sequence data are converted to serial data and one or more optical signals are produced based on the serial data to be transmitted through one or more transmission lines each made of an optical fiber cables.
In the field of video signals, digitalization of video signals has been aimed for actualizing diversification in information to be transmitted, improvements in quality of images reproduced from the video signal and so on. For example, there has been proposed the High Definition Television (HDTV) system which uses a digital video signal composed of digital word sequence data representing video signal information. The digital video signal under the HDTV system (hereinafter, referred to an HD digital video signal) is formed in accordance with, for example, one of a series of standards established by the Broadcasting Technology Association (BTA) in Japan so as to be in the form of Y and PB/PR signals or G, B and R signals. In the case of the Y and PB/PR signals, Y represents a luminance signal and PB/PR represent color difference signals. In the case of the G, B and R signals, G, B and R represent green, blue and red primary color signals, respectively.
The HD digital video signal produced in the form of Y and PB/PR signals is a digital television signal for interlaced scanning by which each frame picture is formed with first and second field pictures each appearing at a rate of 60 Hz or 60/1.001 Hz (hereinafter, the expression “60 Hz” includes both of 60 Hz and 60/1.001 Hz) and produced in accordance with such data formats as shown in FIGS. 1A and 1B.
The data formats shown in FIGS. 1A and 1B include a luminance signal data sequence (Y data sequence) YD(10) as shown in FIG. 1A, which represents a luminance signal component of a video signal, and a color difference signal data sequence (PB/PR data sequence) CD(10) as shown in FIG. 1B, which represents color difference signal components of the video signal. Each of data words constituting the Y data sequence YD(10) or the PB/PR data sequence CD(10) is composed of 10 bits. The word transmission rate of each of the Y data sequence YD(10) and the PB/PR data sequence CD(10) is selected to be, for example, 74.25 MBps. A part of the Y data sequence YD(10) which includes a portion corresponding to a horizontal blanking period and parts of portions corresponding to a couple of video data periods appearing before and after the horizontal blanking period in a horizontal period of the Y data sequence YD(10) is shown in FIG. 1A. Similarly, a part of the PB/PR data sequence CD(10) which includes a portion corresponding to a horizontal blanking period and parts of portions corresponding to a couple of video data periods appearing before and after the horizontal blanking period in a horizontal period of the PB/PR data sequence CD(10) is shown in FIG. 1B.
In the Y data sequence CD(10), time reference code data SAV (Start of Active Video) which are composed of four 10-bit words (3FF(Y), 000(Y), 000(Y), XYZ(Y):3FF and 000 are hexadecimal numbers and (Y) indicates a word contained in the Y data sequence CD(10)) are provided just before a portion corresponding to the video data period and another time reference code data EAV (End of Active Video) which are composed of four 10-bit words (3FF(Y), 000(Y), 000(Y), XYZ(Y)) are provided just after the portion corresponding to the video data period. Similarly, in the PB/PR data sequence CD(10), time reference code data SAV which are composed of four 10-bit words (3FF(C), 000(C), 000(C), XYZ(C): 3FF and 000 are hexadecimal numbers and (C) indicates a word contained in the PB/PR data sequence CD(10)) are provided just before a portion corresponding to the video data period and another time reference code data EAV which are composed of four 10-bit words (3FF(C), 000(C), 000(C), XYZ(C)) are provided just after the portion corresponding to the video data period. The time reference code data EAV and SAV contained in the Y data sequence YD(10) are provided in a portion corresponding to the horizontal blanking period of the Y data sequence YD(10) and the time reference code data EAV and SAV contained in the PB/PR data sequence CD(10) are provided in a portion corresponding to the horizontal blanking period of the PB/PR data sequence CD (10).
The HD digital video signal produced in the form of G, B and R signals is also a digital television signal for interlaced scanning and produced in accordance with such data formats as shown in FIGS. 2A, 2B and 2C. The data formats shown in FIGS. 2A, 2B and 2C include a green primary color signal data sequence (G data sequence) GD(10) as shown in FIG. 2A, which represents a green primary color signal component of a video signal, a blue primary color signal data sequence (B data sequence) BD(10) as shown in FIG. 2B, which represents a blue primary color signal component of the video signal, and a red primary color signal data sequence (R data sequence) RD(10) as shown in FIG. 2C, which represents a red primary color signal component of the video signal. Each of data words constituting the G data sequence GD(10), the B data sequence BD(10) or the R data sequence RD(10) is composed of 10 bits. The word transmission rate of each of the G data sequence GD(10), the B data sequence BD(10) and the R data sequence RD(10) is selected to be, for example, 74.25 MBps. A part of the G data sequence GD(10) which includes a portion corresponding to a horizontal blanking period and parts of portions corresponding to a couple of video data periods appearing before and after the horizontal blanking period in a horizontal period of the G data sequence GD(10) is shown in FIG. 2A. Similarly, a part of the B data sequence BD(10) which includes a portion corresponding to a horizontal blanking period and parts of portions corresponding to a couple of video data periods appearing before and after the horizontal blanking period in a horizontal period of the B data sequence BD(10) is shown in FIG. 2B and a part of the R data sequence RD(10) which includes a portion corresponding to a horizontal blanking period and parts of portions corresponding to a couple of video data periods appearing before and after the horizontal blanking period in a horizontal period of the R data sequence RD(10) is shown in FIG. 2C.
In each of the G data sequence GD(10), the B data sequence BD(10) and the R data sequence RD(10), time reference code data SAV which are composed of four 10-bit words (3FF(G), 000(G), 000(G), XYZ(G):3FF and 000 are hexadecimal numbers and (G) indicates a word contained in the G data sequence DG(10)), time reference code data SAV which are composed of four 10-bit words (3FF(B), 000(B), 000(B), XYZ(B):3FF and 000 are hexadecimal numbers and (B) indicates a word contained in the B data sequence BD(10)) or time reference code data SAV which are composed of four 10-bit words (3FF(R), 000(R), 000(R), XYZ(R):3FF and 000 are hexadecimal numbers and (R) indicates a word contained in the R data sequence RD(10)) are provided just before a portion corresponding to the video data period and another time reference code data EAV which are composed of four 10-bit words (3FF(G), 000(G), 000(G), XYZ(G)), another time reference code data EAV which are composed of four 10-bit words (3FF(B), 000(B), 000(B), XYZ(B)) or another time reference code data EAV which are composed of four 10-bit words (3FF(R), 000(R), 000(R), XYZ(R)) are provided just after the portion corresponding to the video data period. The time reference code data EAV and SAV contained in each of the G data sequence GD(10), the B data sequence BD(10) and the R data sequence RD(10) are provided in a portion corresponding to the horizontal blanking period of each of the G data sequence GD(10), the B data sequence BD(10) and the R data sequence RD(10).
When the HD digital video signal produced, for example, in the form of Y and PB/PR signals for interlaced scanning as described above is subjected to transmission through a data transmission line, the Y data sequence YD(10) and the PB/PR data sequence CD(10) are multiplexed, with their portions corresponding to the horizontal blanking periods in each of which the time reference code data EAV and SAV are provided and which synchronize with each other, to produce a multiplex word sequence and then the multiplex word sequence is converted to serial data to be transmitted. Each of data words constituting the multiplex word sequence is composed of 20 bits and the word transmission rate of the multiplex word sequence is set to be 74.25 MBps. Accordingly, the HD digital video signal produced in the form of Y and PB/PR signals is transmitted in the form of serial data at the bit transmission rate of 74.25×20=1.485 Gbps.
Although, for the present, the HD digital video signal produced in the form of Y and PB/PR signals for interlaced scanning is transmitted in the form of serial data having the bit transmission rate of 1.485 Gbps, as described above, there has been proposed for the future to transmit an HD digital video signal produced in the form of Y and PB/PR signals which is a digital television signal for sequential scanning by which frame pictures are successively obtained without first and second field pictures. In such a case, the HD digital video signal produced in the form of Y and PB/PR signals for sequential scanning, which is usually called a progressive HD digital video signal, is transmitted in the form of serial data having the bit transmission rate of 1.485×2=2.97 Gbps. This bit transmission rate of 2.97 Gbps is twice the bit transmission rate of the serial data based on the HD digital video signal produced in the form of Y and PB/PR signals for interlaced scanning.
Namely, when the progressive HD digital video signal produced in the form of Y and PB/PR signals is subjected to transmission through a data transmission line, the Y data sequence YD(10) and the PB/PR data sequence CD(10), the word transmission rate of each of which is selected to be 74.25×2=148.5 MBps, are multiplexed, with their portions corresponding to the horizontal blanking periods in each of which the time reference code data EAV and SAV are provided and which synchronize with each other, to produce a multiplex word sequence which constitutes a 20-bit word sequence data having the word transmission rate of 148.5 MBps and the multiplex word sequence is converted to serial data to be transmitted. Accordingly, the progressive HD digital video signal produced in the form of Y and PB/PR signals is transmitted in the form of serial data having the bit transmission rate of 148.5 MBps×20=2.97 Gbps.
There has been also proposed a kind of progressive HD digital video signal by which frame pictures are successively obtained at a rate of other than 60 Hz, for example, 50 Hz, or which is aimed for reproducing moving pictures of a cinefilm at twenty-four frames per second with so improved quality as to be substantially equal to that of images reproduced by means of the HDTV system and so-called a D-Cinema signal. Although the D-Cinema signal is able to be obtained in the form of one of the progressive HD digital video signals, the frame rate of which is selected to be, for example, 24 Hz or 24/1.001 Hz (hereinafter, the expression “24 Hz” includes both of 24 Hz and 24/1.001 Hz) as mentioned above, the frame rate of the D-Cinema signal is selected to be not only 24 Hz but also a rate other than 24 Hz, for example, 25 Hz, 30 Hz or 30/1.001 Hz (hereinafter, the expression “30 Hz” includes both of 30 Hz and 30/1.001 Hz).
Such a progressive HD digital video signal produced in the form of Y and PB/PR signals and transmitted in the form of serial data having the bit transmission rate of 2.97 Gbps is of extremely high speed compared with the previous HD digital video signal. Consequently, it is necessary for transmitting the progressive HD digital video signal produced in the form of Y and PB/PR signals to use circuit devices containing respectively a serial to parallel (S/P) convertor which can deal with serial data at an extremely high bit transmission rate, a parallel to serial (P/S) convertor which can deal with serial data at an extremely high bit transmission rate and so on. However, it is impossible to apply previously proposed circuit devices containing respectively an S/P convertor, a P/S convertor and so on to be used as the circuit devices containing respectively the S/P convertor which can deal with serial data having the extremely high bit transmission rate, the P/S convertor which can deal with serial data having the extremely high bit transmission rate and so on. This means that new circuit devices containing respectively the S/P convertor which can deal with serial data having the extremely high bit transmission rate, the P/S convertor which can deal with serial data having the extremely high bit transmission rate and so on, must be developed and such development of the new circuit devices necessitates long time and great cost.
In the situation as mentioned above, it has been proposed to divide the progressive HD digital video signal into two channels of digital data so that circuit devices containing respectively the S/P convertor, the P/S convertor and so on, which have been already put into practice to be used for transmitting the HD digital video signal produced in the form of Y and PB/PR signals for interlaced scanning, can be used for transmitting each of two channels of digital data when word sequence data such as the progressive HD digital video signal are transmitted. According to this proposal, for example, the progressive HD digital video signal produced in the form of Y and PB/PR signals, which constitutes 20-bit word sequence data having the word transmission rate of 148.5 MBps, is divided into two channels of digital data each constituting 20-bit word sequence data having the word transmission rate of 74.25 MBps. In such a case, although two data transmission channels are required, the circuit devices containing respectively the S/P convertor, the P/S convertor and so on, which have been previously developed to be used for transmitting the HD digital video signal produced in the form of Y and PB/PR signals for interlaced scanning, can be used in each of the data transmission channels for transmitting the divided progressive HD digital video signal produced in the form of Y and PB/PR signals.
When such word sequence data as the progressive HD digital video signal produced in the form of Y and PB/PR signals are divided into two channels of digital data and these digital data are transmitted through two data transmission channels, respectively, it is strongly desired that any time difference is not brought about between two channels of digital data transmitted respectively through two data transmission channels so that the original word sequence data can be appropriately reproduced from two channels of digital data received at a receiving side. However, in practice, it is quite difficult to prevent the time difference from bringing about between two channels of digital data transmitted respectively through two data transmission channels for the following reasons.
When two channels of digital data are transmitted respectively through two data transmission channels, each channel of digital data is subjected to various data processings including P/S conversion, S/P conversion and so on in each data transmission channel. As for the P/S conversion or the S/P conversion, for example, variations in logic parameters initially determined in a P/S convertor or S/P convertor conducting the P/S conversion or the S/P conversion of the digital data and variations in processing time in response to a condition of synchronous detection and so on are caused unavoidably. Consequently, time difference resulting from variations in the processing time necessary for the P/S conversion or S/P conversion, for example, and corresponding to, for example, one parallel word period at the maximum is brought about between two channels of digital data transmitted respectively through two data transmission channels.
With regard to transmission of two channels of digital data, there has been previously proposed a circuit for absorbing time difference between two transmitted channels of digital data, as described in, for example, Japanese patent application published before examination under publication number TOKUKAISHO 10-257037. In the circuit thus proposed, two channels digital data are converted respectively into two optical signals having respective central wavelengths different from each other and two optical signals are multiplexed to be transmitted through an optical fiber. The circuit is operative to absorb transmission time difference between two optical signals transmitted through the optical fiber, which results from delay time of each of the optical signals which is brought about in the optical fiber to correspond to the wavelength of the optical signal. In more concrete terms, the delay time of each of the optical signals brought about in the optical fiber is previously presumed based on the central wavelength of the optical signal and a FIFO (first-in first-out) memory device is provided in one of data transmitting channels, in which the shorter delay time of the optical signal is caused. The digital data converted from one of the optical signals transmitted through the optical fiber to have the shorter delay time is stored once in the FIFO memory and then read from the FIFO memory so as to have time delay substantially equal to the time delay of the digital data converted from the other of the optical signals transmitted through the optical fiber. Consequently, the time difference between two channels of digital data converted respectively from the optical signals transmitted through the optical fiber can be absorbed.
This previously proposed circuit for absorbing time difference between two channels of digital data in not able to be applied for absorbing time difference which is caused between two channels of digital data transmitted respectively two data transmission channels by, for example, P/S conversion or S/P conversion to which each of the two channels of digital data are subjected in the respective data transmission channel. The reason of this is that it is impossible to presume previously processing time necessary for the P/S conversion or S/P conversion to which each of the two channels of digital data are subjected in the respective data transmission channel so as to detect one of the channels of digital data provided with the shorter processing time and therefore one of the channels of digital data which is to be stored once in the FIFO memory and then to be read from the FIFO memory can not be determined.
Under such a condition as described above, any practical embodiment of useful system which can effectively absorb time difference which is caused between two channels of digital data, which are obtained by dividing word sequence data such as a progressive HD digital video data and transmitted respectively two data transmission channels, by P/S conversion, S/P conversion or the like to which each of two channels of digital data is subjected in the respective data transmission channel, has not been previously found. Further, any literature or thesis disclosing the useful system which can effectively absorb such time difference as mentioned above has not been previously found also.
Accordingly, it is an object of the present invention to provide a data time difference absorbing circuit which is operative to absorb appropriately and effectively time difference which caused between plural channels of digital data transmitted respectively through plural data transmission channels by, for example, P/S conversion or S/P conversion to which each of the plural channels of digital data is subjected in the respective data transmission channel, a method of receiving digital data in which the data time difference absorbing circuit can be used, and an apparatus for receiving digital data on which the method of receiving digital data is carried out.